Alzheimer's disease (AD), a degenerative and incurable form of dementia, has been correlated with the accumulation of neurofibrillary tangles and amyloid plaques/fibrils, the latter of which are neurotoxic, and comprise Aβ peptide subunits (WO 2007/005359)(Klein et al, 2004, Neur. Biol. Aging 25 (5):569-580). Aβ peptides are typically 39-43 amino acids in length, and are produced by endoproteolysis of the amyloid-β precursor protein (APP) (Bitan et al. (2001) JBC 276 35176-35184). Studies have shown that Aβ42 is the most common monomer subunit found in Aβ plaques of Alheimer's patients (Sanders et al. (2009). Peptides 30, 849-854). Additionally, elevated plasma levels of Aβ42 have been associated with AD (see U.S. 2008/0095706); and increased Aβ42/40 ratios in cerebrospinal fluid (CSF) have been correlated with increased risk of progression from Mild Cognitive Impairment (MCI) to AD (Brys M et al, 2009, Neur. Biol. Aging 30 (5): 682-690.
Under pathological conditions, Aβ peptides undergo conformational changes, from nascent random coil structures to β-sheet conformations (WO 2008/028939). The β-sheet conformation is stabilized by intermolecular hydrogen bonds between individual β-sheets. Conversion to the β-sheet conformation leads to stacking of β-sheets, creating Aβ oligomers and eventually fibrils. Mechanistic studies of Aβ fibril formation suggest that certain soluble assemblies of Aβ, including Aβ oligomers, are intermediates in fibril formation. Additionally, it has been found that the Aβ oligomers are metastable and form spontaneously at low concentrations of Aβ peptide (Lambert et al. (1998). PNAS 95 6448-6453).
Although formation of Aβ fibrils is a pathological feature of AD, the number and distribution of Aβ fibrils and plaques do not always correlate with neurodegeneration or clinical dementia (Dahlgren et al. (2002). JBC, 277 32046-32053). It is therefore unclear whether the accumulation of Aβ fibrils is associated with causation of AD, or is a late-stage manifestation of the disease. Recent evidence has shown that soluble assemblies of Aβ, such as oligomeric assemblies, amyloid-derived diffusible ligands (ADDLs) and Aβ protofibrils, are neurotoxic elements, suggesting a pathological role for Aβ that has not assembled into fibrils.
There are many examples in the AD literature demonstrating that endogenous Aβ oligomers are an important toxic species in AD. For example, Aβ oligomers present in untreated ex vivo human CSF has been shown to rapidly and potently disrupt synaptic plasiticity mechanisms that are believed to underlie memory in the hippocampal network, whereas isolated Aβ monomers had no such effect (Klyubin et al., 2008, J. Neuroscience, 28 (16): 4231-4237). Moreover, soluble Aβ oligomers isolated from AD brains have been shown to disrupt the memory of learned behavior in normal rats (Shankar et al., 2008, Nature Medicine, epub). Studies of multiple strains of APP-transgenic mice show synapse loss, in the absence of fibrillar amyloid deposits, that correlates with levels of Aβ immunoreactivity in soluble extracts (Mucke et al., 2000, J. Neuroscience 20:4050; Lauren et al., 2009, Nature 457: 1128-1132). These findings, however, do not rule out an additional or distinct pathological role for Aβ fibers in development of AD (Kirkitadze et al. (2001). JMB 312 1103-1119; WO 2006/004824).
Synthetically produced oligomers have been shown to have effects similar to those described for naturally occurring oligomers. For example, Aβ42 ADDL's, formed in F12 media, have been shown to specifically inhibit hippocampal long-term potentiation (but don't effect cerebellar neurons) when administered at sub-micromolar doses, and are correlated with synapse loss, effects similar to the activity described for naturally-occuring oligomers in the previous paragraph (Klein, 2002, Neurochemistry Int., 41: 345-352; Klein et al, 2001, Trends Neuroscience, 24:219-224). SDS-derived Aβ42 oligomers also demonstrate specific effects on LTP of hippocampal tissue, as well specifically interacting with dendritic processes of hippocampal neurons (Barghorn et al, 2005, J. Neurochemistry, 95: 834-847).
Aβ monomers and oligomers, including oligomers comprised of Aβ42, are inherently thermodynamically unstable, making it difficult to use the same oligomer composition over an extended period of time. Due to the Aβ peptide's hydrophobic nature, it is favorable for the Aβ peptide to form higher order structures. Accordingly, Aβ peptides form high molecular weight fibrils that readily precipitate from aqueous solution. Although soluble Aβ oligomers have been studied with regard to their neurotoxicity, these studies have been hindered due to the tendency of oligomers to aggregate or combine to form higher order structures.
In an attempt to circumvent the problem of Aβ aggregation, Aβ oligomers are typically stored precipitated or solubilized in PBS or weakly buffered solutions and stored either at 4° C. or frozen. However, precipitated and frozen preparations are typically suitable for short-term usage only, due to their tendency to further aggregate and change their oligomeric characteristics. A stable composition of Aβ oligomers would be useful because it would allow work to be performed on Aβ oligomers without making repeated fresh preparations of oligomer; and reduce variability in oligomer preparations used in a series of investigations. The present invention addresses this and other needs.